1. Field of the Invention
Aspects of the present invention relate to an ink-jet image forming apparatus, and more particularly, to an ink-jet image forming apparatus that can compensate for image quality degradation caused by defective nozzles.
2. Description of the Related Art
An ink-jet image forming apparatus forms images by firing ink from an ink-jet head onto a printing medium. The ink-jet head is placed a predetermined distance apart from the printing medium and reciprocally moves in a direction perpendicular to the transferring direction of the printing medium. Such ink-jet image forming apparatus is referred to as a shuttle type ink-jet image forming apparatus. An ink-jet head of the shuttle type ink-jet image forming apparatus includes a nozzle unit on which a plurality of nozzles that fire ink are formed.
Recently, an ink-jet head having a nozzle unit with a length corresponding to the width of printing medium has been used to realize high-speed printing. An image forming apparatus operating in this way is referred to as a page width ink-jet image forming apparatus. An ink-jet head of the page width ink-jet image forming apparatus is fixed and only a printing medium is transferred. Accordingly, a driving device of the ink-jet image forming apparatus is simple and high-speed printing is possible.
FIG. 1A shows printing patterns when a nozzle unit of a conventional ink-jet image forming apparatus is missing or not working, and FIG. 1B shows dots actually printed by the conventional ink-jet image forming apparatus. FIGS. 2A through 2D are pixel images for explaining a method for compensating for a defective nozzle unit of the conventional ink-jet image forming apparatus. FIG. 3 is a view for explaining a method for compensating for the defective nozzle unit using a secondary ink-jet head in the conventional ink-jet image forming apparatus.
Referring to FIG. 1A, the ink-jet image forming apparatus forms an ink image by firing ink I from nozzles 82 formed in a nozzle unit 80 onto a printing medium. The conventional nozzle unit 80 is installed in the direction perpendicular to the transferring direction of the printing medium, and fires ink onto the printing medium. Thus, when a nozzle 84 is defective, a visible line is generated on the printing medium as shown in FIG. 1A. Such a white line is easily visible, and thus affects printing quality. Further, as shown in FIG. 1B, when the conventional ink-jet image forming apparatus prints a straight line, the line is very uneven due to dot spreading on the printing medium. In other words, since the nozzles of the conventional invention have the same size, the printing quality is deteriorated due to the dot spreading.
A shuttle type ink-jet image forming apparatus prints an image in an overlapping manner by reciprocally moving a carriage several times to compensate for the printing quality deterioration due to a missing or not working nozzle. Such a method is known as a shingling method. In a page width ink-jet image forming apparatus, an ink-jet head is mounted perpendicular to the transferring direction of a printing medium, and does not reciprocally travel in the width direction of the printing medium. That is, there is a specific area on the printing medium on which a nozzle must print an image at a particular moment during printing. If the nozzle does not print an image or malfunctions at a particular moment, a white line is generated on the area where the printing is not performed as illustrated in FIG. 1A, and thus image quality is degraded.
A method for compensating for deterioration of image quality due to a missing or not working nozzle is disclosed in U.S. Pat. No. 5,581,284. FIGS. 2A through 2D are the same drawings illustrated in FIGS. 3 through 6 of U.S. Pat. No. 5,581,284. Referring to FIGS. 2A through 2D, the defective nozzle indicates a case when a nozzle does not normally fire ink, or is missing, in the pixel image 60 as the printing medium 18 is moved in the direction of the arrow 62. When a defective nozzle 63 for firing mono (i.e., black) ink is identified, ink droplets of other colors (i.e., cyan, magenta, and yellow) are sequentially fired to a pixel region 63 to which the defective nozzle should have fired black ink. The black ink was fired correctly in pixels 61. These processes are illustrated in FIGS. 2B, 2C, and 2D. As described above, the black color can be represented by printing the cyan, magenta, and yellow ink droplets on the same location of the printing medium, and the represented black is called process black or composite black. However, this method is useful to compensate for a defect of a nozzle firing black ink, but does not compensate for a defect of any of the nozzles firing other colors. Moreover, since the nozzles for cyan, magenta, and yellow ink do not operate when only the black color is printed, the process black can be formed using the color nozzles. However, when a color image is printed, the nozzles for cyan, magenta, and yellow ink operate, and the compensation cannot be performed. Further, when one of the nozzles used for compensation malfunctions, other colors such as red (yellow+magenta), green (cyan+yellow), or blue (cyan+magenta) color are printed, and thus printing quality is deteriorated.
A method for compensating for deterioration of image quality due to a defective nozzle in an ink-jet printer 10 is disclosed in U.S. Pat. No. 5,587,730. FIG. 3 is the same drawing illustrated in FIG. 2 of the above U.S. patent. The ink-jet printer 10 includes a maintenance station 12, a paper feeder 2, a paper transport 4 and a dryer 6. The method for compensating for a missing dot generated by a failed ink-jet head 20a includes connecting a secondary ink-jet head 20e to the same ink tank 8a as the failed ink-jet head 20a to use the same color and simultaneously operating the secondary ink-jet head 20e and the failed ink-jet head 20a. Using this method, missing dots of all colors due to defective nozzles can be compensated when a plurality of ink-jet heads are used, but the cost of the image forming apparatus increases since the number of ink-jet heads increases. Further, the same color is used to compensate for the missing dot, and therefore, the double ink-jet heads are required to compensate for the missing dots of all colors.